u-boot: Fix repo URL for u-boot-2018.07-at91
u-boot: Fix repo URL for u-boot-2018.07-at91

Signed-off-by: Razvan Stefanescu <f4ee3fcb6e54dcf71fbaefdd500eea5cee1f4f99@microchip.com>

linux4sam/meta-atmel,linux4sam/meta-atmel,alexandrebelloni/meta-atmel,linux4sam/meta-atmel,alexandrebelloni/meta-atmel,alexandrebelloni/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel